The present invention relates to the cooling of turbine shrouds and, more particularly, to an apparatus for the impingement cooling of turbine shrouds as well as a system for flowing a cooling medium, in series, through several cooling cavities of a turbine shroud in a single, closed circuit.
Shrouds in an industrial gas turbine engine are located over the tips of the bucket. The shrouds assist in creating the annulus that contains the hot gas path air used by the buckets to produce rotational motion and, therefore, power. Thus, the shrouds are used to form the gas path of the turbine section of the engine. In advanced gas turbine designs, it has been recognized that the temperature of the hot gas flowing past the turbine components could be higher then the melting temperature of the metal. It is therefore necessary to establish a cooling scheme to protect the hot gas path components during operation.
Typical turbine shrouds are cooled by conduction, impingement cooling, film cooling or combinations of the above. More specifically, one method for cooling turbine shrouds employs an air impingement plate which has a multiplicity of holes for flowing air through the impingement plate at relatively high velocity due to a pressure difference across the plate. The high velocity air flow through the holes strikes and impinges on the component to be cooled. After striking and cooling the component, the post-impingement air finds its way to the lowest pressure sink.
Cooling air usage in a gas turbine is very costly for performance and emissions. However, as noted above, high technology engines produce high firing temperatures and the hot gas path components need to be actively cooled to be able to withstand the high gas path temperatures encountered under these circumstances.
Steam has been demonstrated to be a desired alternative cooling media for cooling gas turbine parts, particularly for combined-cycle plants. However, because steam has a higher heat capacity than the combustion gas, it is inefficient to allow coolant steam to mix with the hot gas stream. Consequently, it is desirable to maintain cooling steam inside the hot gas path components in a closed circuit. Using a closed circuit cooling system achieves the objectives of greater performance with less emissions.
U.S. Pat. No. 5,391,052, the disclosure of which is incorporated herein by this reference, describes apparatuses and methods for impingement cooling of turbine components, particularly turbine shrouds using steam as a cooling medium. U.S. Pat. No. 5,480,281, the disclosure of which is incorporated herein by this reference, provides an apparatus for impingement cooling turbine shrouds in a manner to reduce cross flow effects as well as a system for flowing a cooling medium, in series, through a pair of cooling cavities of the turbine shroud in a single flow circuit. While the apparatuses and methods disclosed in these patents afford effective steam cooling of turbine shrouds, there remains a continuing need for improving turbine shroud cooling while minimizing the amount of cooling media required and reducing cross flow effects.
The present invention provides an improved closed cooling flow circuit for cooling turbine shrouds which provides for flowing a cool medium through a plurality of cooling chambers defined in the cooling cavity of the shroud so as to achieve a series of impingement cooling operations to maximize the cooling of the wall of the shroud exposed to the hot gas path and to minimize detrimental cross flow effects without reducing the area that is subject to impingement cooling.
The closed circuit cooling configuration described hereinbelow may be used with any cooling medium. However, in the presently preferred embodiment, the cooling medium is steam and thus steam will generally be referred to hereinbelow in a non-limiting manner as the cooling medium.
The invention is embodied, therefore, in an apparatus in which steam is brought on board into the outer shroud and spilt so as to be directed to the respective inner shrouds. Within each inner shroud, the steam or other cooling medium is impinged on the shroud inner surface opposite the hot gas path surface of the inner shroud. The post impingement steam flows into a second chamber of the inner shroud to again be impinged on the shroud inner surface for impingement cooling of that portion of the inner shroud. In the presently preferred, exemplary embodiment, the flow of post impingement steam and re-impingement of the inner shroud surface is then repeated through third and fourth chambers of the inner shroud. The spent steam is then returned to the system for being reused in the cycle. The system described hereinbelow is particularly adapted for a combined cycle system installation.
The present invention improves engine performance and reduces engine emissions while still maintaining the program requirements of part life and cost effectiveness.